This invention relates to steam turbines and, more particularly, to an end blade for optimizing performance of a final stage of turbine blading.
The traditional approach to meeting the needs of electric utilities over the years was to build larger units requiring increased exhaust annulus area with successive annulus area increases of about 25%. In this way, a new design with a single double flow exhaust configuration would be offered instead of an older design having the same total exhaust annulus area but with two double flow LP turbines. The newer design would have superior performance in comparison to the old design because of technological advances.
In recent years, the market has emphasized replacement blading on operating units to extend life, to obtain the benefits of improvied thermal performance (both output and heat rate), and to improve reliability and correction of equipment degradation. In addition, the present market requires upgraded versions of currently available turbine designs with improved reliability, lower heat rate and increased flexibility.
The latter stages of the steam turbine, because of their length, produced the largest proportion of the total turbine work and therefore have the greatest potential for improved heat rate. The last turbine stage operates at variable pressure ratio and consequently the stage design is extremely complex. All of the first turbine stage, if it is a partial-arc admission design, experiences a comparable variation in operating conditions. In addition to the last stage, the upstream low pressure (LP) turbine stages can also experience variations on operating conditions because of: (1) differences in rated load end loading; (2) differences in site design exhaust pressure and deviations from the design values; (3) hood performance differences on various turbine frames; (4) LP inlet steam conditions resulting from cycle steam conditions and cycle variations; (5) location of extraction points; (6) operating load profile (base load versus cycle); and (7) zoned or multi-pressure condenser applications versus unzoned or single pressure condenser applications. Since the last few stages in the turbine are tuned, tapered, twisted blades with more selected inlet angles, the seven factors identified above have greater influence in stage performance. Consequently, it is desirable to design last row blades for low pressure steam turbines in a manner to meet the requirements of the above listed seven factors.